1. Technical Field of the Invention
The present invention relates generally to a motor control apparatus controlling the operation of a brushless motor through an inverter circuit, and more particularly to a motor control apparatus designed to monitor a malfunction in the apparatus.
2. Background Art
Motor control apparatus are known in the art which are designed to control the operation of a brushless motor through an inverter circuit made up of bridged power switching elements and to control the speed of the motor using PWM (Pulse-Width- Modulated) signals.
A typical motor control apparatus of the above type will be described with reference to FIGS. 6 and 7. FIGS. 6 and 7 show an example where a motor 1 includes three windings (stator coils) L1, L2, and L3 having first, second, and third phases (also referred to as U, V, and W phases below) which are connected in a delta form.
The motor control apparatus, as shown in FIG. 6, includes a switching circuit 6 that is a three-phase bridge circuit made up of positive switching elements 3p, 4p, and 5p and negative switching elements 3n, 4n, and 5n. The positive switching elements 3p to 5p are connected between a positive terminal (i.e., a higher voltage side) of a dc power supply 2 and terminals Ju, Jv, and Jw of the windings L1 to L3. Similarly, the negative switching elements 3n to 5n are connected between a negative terminal (i.e., a lower voltage side) of the power supply 2 and the terminals Ju, Jv, and Jw of the windings L1 to L3. The switching elements 3p, 4p, 5p, 3n, 4n, and 5n are each made by an N-channel MOSFET.
The motor control apparatus also includes a commutation control circuit 7 which provides drive signals Up, Un, Vp, Vn, Wp, and Wn in response to rotor position indicative signals (not shown) of the motor 1, respectively, to turn on the switching elements 3p, 3n, 4p, 4n, 5p, and 5n in a given sequence. The commutation control circuit 7 forms an inverter circuit 8 together with the switching circuit 6.
The motor control apparatus also includes a PWM signal generator 9 and AND gates 13, 14, and 15. The PWM signal generator 9 provides a PWM signal for controlling the speed of the motor 1. The AND gates 13 to 15 receive the drive signals Un, Vn, and Wn outputted from the commutation control circuit 7 and the PWM signal outputted from the PWM signal generator 9.
The drive signals Up, Vp, and Wp produced by the commutation control circuit 7 are inputted to gates of the switching elements 3p, 4p, and 5p, respectively. Outputs of the AND gates 13, 14, and 15 are inputted to gates of the switching elements 3n, 4n, and 4n, respectively.
Each of the switching elements 3p, 4p, and 5p is turned on when a corresponding one of the drive signals Up, Vp, and Wp is at a higher level, while each of the switching elements 3n, 4n, and 5n is turned on when a logical product of the PWM signal from the PWM signal generator 9 and a corresponding one of the drive signals Un, Vn, and Wn is at the higher level (see FIG. 7).
The commutation control circuit 7 switches, as shown in FIG. 7, the drive signals Up, Un, Vp, Vn, Wp, and Wn between the high level and the low level in a cycle to rotate the motor 1. In FIG. 7, each of the drive signals Un, Vn, and Wn to be inputted to the switching elements 3n, 4n, and 5n lying in level on a lower side, as viewed in the drawing, shows the high level. The same is true for FIG. 2, as referred to later.
The positive switching element 3p and the negative switching element 3n connected to the U-phase winding terminal Ju are each turned on for a time period equivalent to an electric angle of 120xc2x0, and become active alternately at a time interval of 60xc2x0 away from each other. The positive switching element 4p and the negative switching element 4n connected to the V-phase winding terminal Jv and the positive switching element 5p and the negative switching element 5n connected to the W-phase winding terminal Jw become active at a phase interval of 120xc2x0 away from the positive switching element 3p and the negative switching element 3n. Specifically, during a time when the positive and negative switching elements connected to one of the phase winding terminals Ju, Jv, and Jw are both turned off (i.e., an electric angular interval of 60xc2x0), another one of the positive switching elements connected to the second one of the phase winding terminals Ju, Jv, and Jw is turned on, while one of the negative switching elements connected to the third one of the phase winding terminals Ju, Jv, and Jw is turned on according to the PWM signal.
FIGS. 6 and 7 illustrate for the case where the negative switching elements 3n, 4n, and 5n are PWM-controlled when the drive signals Un, Vn, and Wn are at the high level, but the positive switching elements 3p, 4p, and 5p, or all the positive and negative switching elements 3n, 4n, 5n, 3p, 4p, and 5p may be PWM-controlled.
If a malfunction occurs in one of the switching elements of the inverter circuit 8 due to a short, turning on of the malfunctioning switching element and a mating one of the switching elements causes an excess current to flow through them. For example, in a case where the U-phase negative switching element 3n is short-circuited, when the U-phase positive switching element 3p is turned on, it will cause an excess current to flow through both the switching elements 3n and 3p. 
In order to detect such a malfunction, the motor control apparatus shown in FIG. 6 has a current detecting shunt resistor 16 disposed between sources of the negative switching elements 3n, 4n, and 4n and the minus (xe2x88x92) terminal of the dc power supply 2 to monitor the potential difference between terminals of the shunt resistor 16. When the potential difference becomes greater than a given value, the motor control apparatus determines that any one of the switching elements has been short-circuited and forces all the switching elements into an off-state (as taught in Japanese Patent First Publication Nos. 6-209581 and 7-274580.
Such a system, however, detects occurrence of a malfunction of a switching element after a mating switching element is turned on so that the current flows through the malfunctioning switching element and the mating switching element and has the disadvantage that the current flow may result in a malfunction of the mating switching element that is normally operating. The system has the further disadvantage that the current flows through the shunt resistor 16 even when the system is normally operating, thereby resulting in loss of electric energy.
Japanese Patent First Publication No. 2-266891 teaches a system which determines that a malfunction has occurred when the voltage appearing at winding terminals of a motor does not change cyclically during an operation of the motor and disables all switching elements. The system, however, has also the disadvantages that it is impossible to detect occurrence of the malfunction before the current flows through the switching elements and the current flow may lead to an additional malfunction.
The above Japanese Patent First Publication No. 7-274580 is also designed to compare the voltage at each of the winding terminals Ju, Jv, and Jw with a reference voltage immediately after switching of a corresponding one of the drive signals Up to Wn, determine that a malfunction has occurred when a result of the comparison does not match up with the one of the drive signals Up to Wn, and disables all the switching elements 3p to 5n. This system, however, the disadvantage that in a case where any one of the switching elements is short-circuited during a time from detection of the malfunction (i.e., the preceding commutation) to the subsequent commutation, a mating one of the switching elements may be short-circuited when turned on. For example, if the U-phase negative switching element 3n is short-circuited during a time from when the drive signal Un, as shown in FIG. 7, changes from high to low level to when the drive signal Up changes from low to high level, the U-phase positive switching element 3p is turned on at the time of the next commutation before the system detects the short circuit of the switching element 3n, which may cause the switching element 3p to be also short-circuited.
The system, as taught in Japanese Patent First Publication No. 7-274580, operates each of the switching elements 3p to 5n when a corresponding one of the drive signals Up to Wn is at high or low level, so that the voltages Vu, Vv, and Vw at the winding terminals Ju, Jv, and Jw depend upon the level of the drive signals Up to Wn. However, since either or both of a set of the positive switching elements 3p to 5p and a set of the negative switching elements 3n to 5n are PWM-controlled, the on-off operations of the switching elements 3p to 5n are not always synchronous with the drive signals Up to Wn, which will result in a difficulty in detecting a failure of the switching elements 3p to 5n accurately at all times.
It is therefore a principal object of the present invention to avoid the disadvantages of the prior art.
It is another object of the present invention to provide a motor control apparatus which is capable of minimizing the damage of the apparatus when a failure occurs in the apparatus.
According to one aspect of the invention, there is provided a motor control apparatus which comprises: (a) an inverter circuit including a switching circuit and a commutation control circuit, the switching circuit including a first set of switching elements each of which is connected at one end to a higher voltage side of a dc power supply and at the other end to one of terminals of windings of a multi-phase motor and a second set of switching elements each of which is connected at one end to a lower voltage side of the dc power supply and at the other end to one of the terminals of the windings of the multi-phase motor, the commutation control circuit producing drive signals to turn on the switching elements in sequence for rotating the motor; (b) a PWM signal generator generating PWM (Pulse-Width-Modulated) signals which turn on at least one of the first and second sets of the switching elements cyclically for controlling the speed of the motor; (c) a switching element failure detecting circuit determining whether a voltage appearing at one of the terminals of the windings of the motor is lower than a given reference voltage or not every active time when the PWM signal is at an active level to turn on the switching elements, when the voltage is lower than the given reference voltage, the switching element failure detecting circuit determining that one of the second set of the switching elements connected to the one of the terminals of the windings has failed and outputting a switching element failure indicative signal; and (d) a switching operation prohibiting circuit responsive to the switching element failure indicative signal outputted from the switching element failure detecting circuit to prohibit one of the first set of the switching elements connected to the one of the terminals of the windings of the motor from being turned on.
In the preferred mode of the invention, the switching element failure detecting circuit includes a comparator and a latch circuit. The comparator compares the voltage appearing at the one of the terminals of the windings of the motor with the given reference voltage to provide an output signal at one of a high and a low level when the voltage is lower than the given reference voltage. The latch circuit includes a gate terminal to which the PWM signal is inputted, a data terminal to which an output signal of the comparator is inputted, and an output terminal which outputs the output signal of the comparator as it is when the PWM signal is at the active level and outputs the output signal of the comparator latched at a time when the PWM signal has changed from the active level to a passive level turning off the switching elements when the PWM signal is at the passive level. The switching operation prohibiting circuit prohibits the one of the first set of the switching elements from being turned on when a signal outputted from the output terminal of the latch circuit is at the one of the high and low level of the output signal of the comparator.
A second switching element failure detecting circuit is further provided which determines whether a voltage appearing at one of the terminals of the windings of the motor is higher than a given reference voltage or not every active time, determines that one of the first set of the switching elements connected to the one of the terminals of the windings has failed to output a second switching element failure indicative signal when the voltage is higher than the given reference voltage. A second switching operation prohibiting circuit is provided which is responsive to the second switching element failure indicative signal to prohibit one of the second set of the switching elements connected to the one of the terminals of the windings of the motor from being turned on.
A failure indicative signal generator may also be provided which generates a failure indicative signal when either or both of the switching element failure detecting circuit and the second switching element failure detecting circuit determine that failure has occurred in the switching element for prohibiting all the switching elements from being turned on.
According to the second aspect of the invention, there is provided a motor control apparatus which comprises: (a) an inverter circuit including a switching circuit and a commutation control circuit, the switching circuit including a first set of switching elements each of which is connected at one end to a higher voltage side of a dc power supply and at the other end to one of terminals of windings of a multi-phase motor and a second set of switching elements each of which is connected at one end to a lower voltage side of the dc power supply and at the other end to one of the terminals of the windings of the multi-phase motor, the commutation control circuit producing drive signals to turn on the switching elements in sequence for rotating the motor; (b) a PWM signal generator generating PWM (Pulse-Width-Modulated) signals which turn on at least one of the first and second sets of the switching elements cyclically for controlling the speed of the motor; (c) a switching element failure detecting circuit determining whether a voltage appearing at one of the terminals of the windings of the motor is higher than a given reference voltage or not every active time when the PWM signal is at an active level to turn on one of the switching elements, when the voltage is higher than the given reference voltage, the switching element failure detecting circuit determining that one of the first set of the switching elements connected to the one of the terminals of the windings has failed and outputting a switching element failure indicative signal; and (d) a switching operation prohibiting circuit responsive to the switching element failure indicative signal outputted from the failure detecting circuit to prohibit one of the second set of the switching elements connected to the one of the terminals of the windings of the motor from being turned on.
In the preferred mode of the invention, the switching element failure detecting circuit includes a comparator and a latch circuit. The comparator compares the voltage appearing at the one of the terminals of the windings of the motor with the given reference voltage to provide an output signal at one of a high and a low level when the voltage is lower than the given reference voltage. The latch circuit includes a gate terminal to which the PWM signal is inputted, a data terminal to which an output signal of the comparator is inputted, and an output terminal which outputs the output signal of the comparator as it is when the PWM signal is at the active level and outputs the output signal of the comparator latched at a time when the PWM signal has changed from the active level to a passive level turning off the switching elements when the PWM signal is at the passive level. The switching operation prohibiting circuit prohibits the one of the first set of the switching elements from being turned on when a signal outputted from the output terminal of the latch circuit is at the one of the high and low level of the output signal of the comparator.
A second switching element failure detecting circuit is provided which determines whether a voltage appearing at one of the terminals of the windings of the motor is lower than a given reference voltage or not every active time and determines that one of the second set of the switching elements connected to the one of the terminals of the windings has failed to output a second switching element failure indicative signal when the voltage is lower than the given reference voltage. A second switching operation prohibiting circuit is provided which is responsive to the second switching element failure indicative signal to prohibit one of the first set of the switching elements connected to the one of the terminals of the windings of the motor from being turned on.
A failure indicative signal generator may also be provided which generates a failure indicative signal when either of both of the switching element failure detecting circuit and the second switching element failure detecting circuit determine that failure has occurred in the switching element for prohibiting all the switching elements from being turned on.
According to the third aspect of the invention, there is provided a motor control apparatus which comprises: (a) an inverter circuit including a switching circuit and a commutation control circuit, the switching circuit including a first set of switching elements each of which is connected at one end to a higher voltage side of a dc power supply and at the other end to one of terminals of windings of a multi-phase motor and a second set of switching elements each of which is connected at one end to a lower voltage side of the dc power supply and at the other end to one of the terminals of the windings of the multi-phase motor, the commutation control circuit producing drive signals to turn on the switching elements in sequence for rotating the motor; (b) a PWM signal generator generating PWM (Pulse-Width-Modulated) signals which turn on at least one of the first and second sets of the switching elements cyclically for controlling the speed of the motor; and (c) a failure detecting circuit monitoring a voltage appearing at one of the terminals of the windings of the motor every active time when the PWM signal is at an active level to turn on one of the switching elements, the failure detecting circuit checking the voltage monitored when the PWM signal, provided to either of one of the first set of the switching elements connected to the one of the terminals and one of the second set of the switching element connected to the one of the terminals, is at the active level to detect a failure in the other switching element.
According to the fourth aspect of the invention, there is provided an apparatus for detecting a motor current flowing through delta-connected armature windings of a three-phase brushless dc motor when the motor is operated through a motor driver. The apparatus comprises: (a) a neutral voltage detecting circuit including three resistors which are connected at one end to the armature windings of the motor, respectively, and at the other end to a common junction, the neutral voltage detecting circuit measuring a voltage developed at the common junction as a neutral voltage V1 of the motor; (b) a speed proportional voltage generator generating a voltage V2 proportional to a speed N of the motor according to an equation (1) below; and (c) a current determining circuit determining a voltage Vo according to an equation (2) below which indicates the motor current flowing through the motor.
V2=Axc2x7Nxe2x80x83xe2x80x83(1)
where A is a constant of proportion which is given by a relation of A =kxc2x7xcfx86/2 where k is a constant of electromotive force and xcfx86 is magnetic flux over one of the armature windings.
Vo=VBxe2x88x92V1xe2x88x92V2xe2x80x83xe2x80x83(2)
where VB is a voltage supplied to the motor driver to operate the motor.
In the preferred mode of the invention, an operation suspending circuit is provided which suspends the operation of the motor for a preselected period of time when the voltage Vo exceeds a given threshold value.
The operation suspending circuit may stop the operation of the motor completely when the number of times the operation of the motor is suspended reaches a preselected number.